Smart heating with Zigbee TRV with an old on/off heater

I have been searching an reading for weeks, months to find a good solution. I can’t seem to find it. So I kinda have my own idea that sound doable. If anybody has advice or tips?

Current situation: I have an old oil burner (Viessmann Vitola Uniferral 4702) with a wired thermostat Theben Ram 784. So this is an on/off system. The thermostat is fixed on the wall in the kitchen. At this moment the kitchen is warm, but other rooms are cold. While I don’t need the kitchen to be warm.

So, I would need a a smart wireless thermostat that I can place in the room where I am. Next to that I would need smart TRV’s, preferably Zigbee, so I can use it on MQTT. If I want to heat the bathroom half hour before my wake up alarm, I would be able to only heat that room.

I looked at Tado, Netatmo, Bosch, Hombli… Only Tado and Netatmo seems to be able to connect on on/off heaters. I didn’t found any solution for the others. But then they don’t have Zigbee TRV’s and I have to connect HA with their ecosystem, which I don’t like.

Now, my idea would be this:
I use Zigbee TRV’s (tuya, bosch, sonoff) in all rooms. I remove the old on/off thermostat and connect a Zigbee Relay (on/off) on the 2-way cable of the old thermostat. This way I can send the command with HA to the relay that puts the heater on and off, based on the temperature settings of the rooms. Since I have the temperature of each room and I can set the wanted temperature, I can automate everything in HA.

Is that such a bad idea? It seems so cheap, so efficient, so easy to set up.
about 20-30€ for a TRV and 20€ for a relay. So less than 200€ instead of the 600+€ if I use Tado.

(It might be smart to avoid using the relay and use a wireless thermostat that has an integration with HA. I can use this thermostat in the living room, so I don’t need 3 TRV’s for that room. And I can set it locally if I have a problem with my HA.)

What do you think?

One problem I see with smart thermostats is that they want 24v power to auto-detect if you support cooling or heating or have multiple stages, etc. That means you put one on the wall and it doesn’t let you change set points because, as far it is concerned, you don’t have anything to control. Some of the lesser known models might work, but I know Nest and Honeywell auto-detect the 24v power coming in.

You could overcome this by faking the 24v power from a relay and that’s probably not a bad idea to have these “fake” thermostats that work like everyone expects them to and just trigger automations for when they are changed.

You could also go the relay route and get the Sonoff NSPanel to build your own thermostat interface. In fact, the Sonoff, out of the box, has a “thermostat” built into it but I’ve never used it - however you can flash it with ESPHome and do your own thing too.

That NSPanel in combination with a relay would look stunning. It has indeed a built in thermostat as well.

I found another option instead of the relay system: Tuya Zigbee Thermostat
For only 39€ I can connect this battery powered Zigbee thermostat on the two wired system of my CV. So I won’t need the relay ànd I can use it wireless in the living room.

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That’s a pretty cool thermostat, I might have to look deeper into that for one of my projects.

just a headsup:

I installed the Tuya Zigbee thermostat. But was mistaken; I couldn’t use it as a wireless thermostat, since off course it’s wired. So at this moment I still have the issue that my kitchen temperature defines if the heater is on or not.

I can already set temperature through HA, which is a much better than before.
But my first idea remains standing: I’m looking into a Zigbee relay that powers on the heater when one of the smart TRV’s is requesting it to be put on.
Something like this

This is really all the netatmo setup does IMO and is exactly what I am going to do. I already have a POC using a zbmini switch (says it supports up to 10A) and a sonoff trv, you can create an automation when the entity changes the action status to heating … then switch on the boiler demand. The automation will get slightly more complex to only turn it off if there are no more valves (for the case where there are N valves) with any demand left I haven’t looked at that yet.

The bigger problem is improving the accuracy of these TRV valves it seems that really what you need is a zigbee room thermostat in each room to completely replace the temperature readings on the valve which is nearly always in a compromised position for temperature readings.

The sonoff TRV I have in my POC is vertically mounted and shoots up tp 24 degrees within 5 or 10 minutes vs the room thermostat will still say 19. I have just put a custom heat reflector underneath it (like its going to the moon or something) and will see if that improves matters. I am not going to change my pipework to mount them horizontally!

edit: I should clarify I have a UK central heating system, worcester bosch, not particularly new. I also have Netatmo and I plan to replace it with sonoff / HA. In this setup I will still have a honeywell timer doing the HW, and a relay the CH demand (I just set the timer to have CH permanently on which just leaves the relay in charge, as we do today for netatmo). But if its all working OK then the honeywell timer can come out too and another relay can drive the hot water!

I am also interested in getting good telemetry and analytics so I can spot early when for example a motorised valve head is starting to fail. Normally you only notice when you have no hot water then someone says helpfully “oh yeah I did notice it ran out quicker this week but forgot to mention”…

edit: just came across the BetterThermostat plugin for HA… it warrants some experimenting!

Instead of a smart thermostat you could control the on/off with a smart relay switch, install a separate (powered) temperature sensor, then combine these using Generic Thermostat. I actually have two temperature sensors at critical locations, so if one fails I can switch to the other.

To schedule this thermostat and your TRVs by calendar, grouped by room etc., with manual override, and optionally with door or window open and presence detection, you might like to check out Heating X2: Schedule Thermostats with Calendars